1. Field of the Invention
This invention generally relates to aircraft surface measurements, and more particularly, to a system for and a method of monitoring the free play of aircraft control surfaces.
2. Description of the Prior Art
All aircraft include movable control surfaces for directional control during flight. Such control surfaces, particularly those in military and commercial aircraft, must be monitored throughout their life, for, amongst other things, the free play of the control surfaces. Free play in control surfaces is an important consideration in maintaining control of the aircraft. Excessive control surface free play can result in flutter, divergence, and other dynamic and aeroelastic instabilities. Free play can also effect control surface vibrations and natural frequencies of structures. Since increasing levels of free play can result in increased fatigue failure of control surfaces, both the military and the FAA have issued standards that outline the amount of free play that is allowed on different aircraft control surfaces for the service life of such aircraft. Additionally, the standards set predetermined points throughout the service life of an aircraft when free play must be tested and monitored to document wear and assure that predetermined specifications can be met.
The current industry standard for measuring aircraft control surface free play requires a static free play test. This is typically done by applying a known load to the control surface and measuring the corresponding deflection, either a linear measurement or angular displacement. For example, a Rotation Variable Differential Transformer (RVDT) is used to measure the rotation between a rudder and a tail of an aircraft, while a test hydraulic actuator and a load cell are used to record the load. Such setups are very labor intensive and can take a considerable amount of time. Furthermore, larger aircraft, such as the Boeing™ C-17™ and Boeing™ 747™ require a tester to maneuver heavy test equipment (load cells, RVDT, hydraulic actuators and their plumbing) up to and down from great heights to measure rudder and stabilator free play. This can sometimes cause delays, damage to the aircraft or other problems. As used herein, all occurrences of BOEING, 747, and C-17 are trademarks or registered trademarks of Boeing Management Company.
Normally, the static free play test is started at zero load and increased to some percentage of ultimate load. Large hydraulic cylinders are required to apply such loads and place the aircraft being tested at risk of being damaged. During testing, the moment or applied load is plotted versus displacement. For a surface with no free play and a linear spring stiffness, this plot is a straight line, with the slope of this measured line being the effective spring stiffness. As free play is introduced into the system, a discontinuity in the curve occurs near the zero load range. For larger displacement values the slope increases, and is more representative of the effective stiffness without the free play. As hysteresis is introduced into the system, the load versus displacement forms a known type curve.
No current system or method is known for dynamically measuring the free play of aircraft control surfaces. Therefore, there is a long felt need in the art for a system and/or a method for dynamically measuring and monitoring the free play of aircraft control surfaces, without incurring the risk of damaging the tested aircraft, and which overcomes other known problems, while allowing easier and less time-consuming monitoring of aircraft control surface free play.